It can be done but it's the same as asking how can one start a faithful monogamous relationship. First you have to find another person who shares your goals, desires, and ambitions. Then you have to build your foundation and begin building your life. There really isn't any difference if you add a third person to the mix.

Again being faithful means abiding by the limits and boundaries agreed upon by all interested parties. If, in your relationship, having other lovers is within your boundaries agreed upon then you are not being unfaithful to sleep with others.

A closed relationship is one where the boundaries include not allowing others into the relationship in a sexual manner (or whatever the group dynamic needs) It is no harder to achieve than a faithful monogamous relationship, and we all know that you can have more than one of those in a lifetime.

You most certainly can force the issue and be choosy when looking to add a third or fourth to your marriage. You were choosy when looking for your first mate why would that change when looking for another?

Most people would agree that relationships can just happen regardless of their magnitude or worth. Poly relationships are no different. Know what you are looking for and what you are willing to compromise on. Open yourself and your relationship to the possibility of finding what you are looking for and then work on making your current relationship some place another person would feel comfortable in.

I most certainly call it an open relationship if a married couple are looking for another like minded individual or individuals to add to their lives. They are "open" to the possibility and have an "Open" and hopefully welcoming marriage. If you are totally sure about what you want, and are attractive to the sort of person you are looking for then chances are you will get what you ask for.